feat(F-G-H-I): STL cache, invoices, import validation, notification settings

Phase F — STL Hash Cache:
- Migration 041: step_file_hash column on cad_files
- cache_service.py: SHA256 hash + MinIO-backed STL cache (check/store)
- render_step_thumbnail: compute+persist hash before render
- generate_stl_cache: check MinIO cache before cadquery conversion, store after

Phase G — Invoices:
- Migration 042: invoices + invoice_lines tables with RLS
- Invoice/InvoiceLine models + schemas
- billing service: generate_invoice_number (INV-YYYY-NNNN), create/list/get/delete/PDF
- WeasyPrint PDF generation; backend Dockerfile + pyproject.toml deps
- invoice_router with 6 endpoints; registered in main.py
- frontend: Billing.tsx page + api/billing.ts; route + nav link

Phase H — Import Sanity Check:
- Migration 043: import_validations table
- ImportValidation model + schemas
- run_sanity_check: material fuzzy-match (cutoff=0.8), STEP availability, duplicate detection
- validate_excel_import Celery task (queue: step_processing)
- uploads.py: create ImportValidation on /excel, fire task, expose GET /validations/{id}
- frontend: Upload.tsx polling ValidationDialog with Ampel status indicators

Phase I — Notification Settings:
- Migration 044: notification_configs table (user×event×channel toggles)
- NotificationConfig model + seeds (in_app=true, email=false)
- get/upsert/reset config endpoints on /notifications/config
- frontend: NotificationSettings.tsx page + api/notifications.ts extensions

Infrastructure:
- docker-compose.yml: add worker-thumbnail service (concurrency=1, Q=thumbnail_rendering)
- Fix Dockerfile: libgdk-pixbuf-2.0-0 (correct Debian bookworm package name)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/api/routers/notifications.py

@@ -155,3 +155,39 @@ async def mark_one_read(
     )
     await db.commit()
     return {"ok": True}
+
+
+# ── Notification Config Endpoints ────────────────────────────────────────────
+from app.domains.notifications.schemas import NotificationConfigOut, NotificationConfigUpdate
+from app.domains.notifications.service import (
+    get_notification_configs, upsert_notification_config, reset_notification_configs
+)
+
+
+@router.get("/config", response_model=list[NotificationConfigOut])
+async def get_my_notification_config(
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    return await get_notification_configs(db, current_user.id)
+
+
+@router.put("/config/{event_type}/{channel}", response_model=NotificationConfigOut)
+async def update_my_notification_config(
+    event_type: str,
+    channel: str,
+    body: NotificationConfigUpdate,
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    if channel not in ("in_app", "email"):
+        raise HTTPException(status_code=400, detail="channel must be 'in_app' or 'email'")
+    return await upsert_notification_config(db, current_user.id, event_type, channel, body.enabled)
+
+
+@router.post("/config/reset", response_model=list[NotificationConfigOut])
+async def reset_my_notification_config(
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    return await reset_notification_configs(db, current_user.id)